This present invention relates to methods and devices that detect DTMF tones. In particular, the invention relates to using cost functions to detect DTMF tones.
Dual Tone Multi-Frequency (DTMF) detectors have become widely used in the telecommunication industry. DTMF signals include two tones, one from a row group of frequencies and one from a column group of other frequencies. A pair of frequencies (one from the row and one from the column) determine a symbol. In one illustrative example, four frequencies may be selected for the row group and four frequencies may be selected for the column group. Sixteen pairs can be created from this grouping and can represent sixteen symbols, for instance, 1, 2, 3, 4, 5, 6, 7, 8, 9, 0, a, b, c, d, *, and #. The row frequencies may be 697 Hz, 770 Hz, 852 Hz, and 941 Hz. The column frequencies may be 1209 Hz, 1336 Hz, 1477 Hz, and 1633 Hz.
Devices in modern telecommunication systems may use several different methods to detect DTMF tones. For example, one technique uses discrete Fourier transforms (DTFs) to detect DTMF tones. Specifically, the DTF values at only the tone frequencies are computed as specified in a modified Goertzel algorithm. Although this technique may detect the DTMF tones, it has certain limitations. For instance, the technique depends upon the use of finely tuned thresholds and, therefore, may have variations depending upon different operating environments.
The system and method of the present invention advantageously allows DTMF tones to be determined by using the values of cost functions. Specifically, an initial determination of the two frequencies of a DTMF tone is made using the values of cost functions. The initial determination of the frequencies is confirmed by re-computing the values of the cost functions using an adjustment factor.
In one embodiment of the present invention, a system for determining DTMF tones includes a linear predictive coding (LPC) analysis module, a cost function generation module, a predetermination module, and an enhancement module.
The LPC analysis module receives an input signal. The input signal includes a DTMF tone. The DTMF tone has a first frequency and a second frequency. Using the input signal, the LPC analysis module computes LPC coefficients using the input signal. The LPC analysis signal may use an equation to model the spectrum of the input signal. The order of this equation can take on any value, for example, 10.
The cost-function generation module is coupled to the LPC analysis module and computes cost functions based, at least in part, upon said LPC coefficients determined by the LPC analysis module.
The predetermination module is coupled to the cost function analysis module and the LPC analysis module. The predetermination module determines an initial first frequency and an initial second frequency by using the cost functions. For example, the predetermination module may compute the values of the cost functions at a plurality of frequencies.
The enhancement module is coupled to the predetermination module and the cost-function analysis module. The enhancement module confirms that the initial first frequency is the first frequency and the initial second frequency is the second frequency based upon re-computing the values cost functions. The cost-functions may be re-computed for a plurality of frequencies and may take into account at least one adjustment factor.